A gas-phase catalytic oxidation reaction by which propane, propylene or isobutylene is, for example, oxidized with molecular oxygen or a gas containing molecular oxygen in the presence of a composite oxidation catalyst to produce (meth)acrolein or (meth)acrylic acid is a typical gas-phase catalytic reaction using a multi-tubular reactor to which this invention pertains.
A multi-tubular reactor used for the reaction of propylene, etc. contains several thousand to several tens of thousand reaction tubes and it is desirable that as uniform a differential pressure as possible act on the reaction tubes at the time of the reaction. If the reaction tubes differ in differential pressure from one another, a problem arises in that a different amount of gas flows from one reaction tube to another and makes a different state of reaction from one reaction tube to another even in one and the same reactor.
The reaction temperature of a reactor is determined in accordance with the average of states of reaction in all the reaction tubes and in the case of, for example, a former-stage reactor intended for the oxidation reaction of propylene, the temperature of the heat medium to be used is usually determined in accordance with the average of the conversion rates of propylene in all the reaction tubes, since the conversion rate of propylene differs from one reaction tube to another. Thus, all of the reaction tubes are not operated under the optimum conditions.
It is important for the safe operation of a multi-tubular reactor to unify the states of reaction in its reaction tubes, or unify their differential pressures, since the state of reaction differing from one reaction tube to another presents the problems as stated below.
(1) At the same reaction temperature, a reaction tube having a large amount of gas shows a low conversion rate of a raw material and a low yield. At the same reaction temperature, a reaction tube having a small amount of gas, on the other hand, shows an excessive reaction with a lot of side reactions and a low selectivity.(2) Moreover, a reaction tube having a small amount of gas shows a deficiency of oxygen at its outlet, causing not only the deterioration of a catalyst, but also its coking, in addition to the occurrence of the excessive side reactions and the lowering of selectivity as stated above.(3) The difference in the state of reaction from one reaction tube to another leads to a different level of deterioration of the catalyst and a shortening of catalyst life as a whole.
However, the reaction tubes showing an abnormal differential pressure, as thereby causing coking, are scattered in a multi-tubular reactor containing several thousand to several tens of thousand reaction tubes as mentioned before and exist without showing any tendency at those sites in the reactor which cannot be explained by the flow of reaction gas or the flow of a heat medium.
There is no technique aimed at unifying the differential pressures of the reaction tubes in a multi-tubular reactor, as far as the inventors of this invention know.